Active pixel circuit for a time-of-flight system and method for operating the same

Abstract

A pixel circuit for performing Time of Flight measurements comprises at least one optical sensor arranged for receiving a reference modulation signal and a light signal and arranged for outputting a photocurrent signal depending on the light signal and on a phase shift corresponding to a phase difference between the light signal and the reference modulation signal, and an integrator circuit comprising an integration capacitor, an amplifier, and switching mean. The switching means is arranged for resetting the integration capacitor in a reset mode, for connecting the integration capacitor between the at least one optical sensor and a voltage reference signal in a passive mode. The negative feedback loop is fed with the photocurrent signal of the at least one optical sensor, and for connecting a signal output by the integrator circuit to an output bus in a readout mode.

Description

FIELD OF THE INVENTION[0001]The present invention is generally related to the field of time-of-flight systems. More in particular it relates to solutions for increasing the dynamic range of such systems.BACKGROUND OF THE INVENTION[0002]Time-of-Flight (ToF) is a method for measuring the distance between a sensor and an object, based on the time difference between the emission of a signal and its return to the sensor, after being reflected by an object. Various types of signals (also called carriers) can be used with ToF, a very common one being light.[0003]A conventional scheme of a ToF pixel is shown in FIG. 1. It uses three transistors SEL, SF and RT and a capacitance C to perform readout of the ToF photocurrent flowing out of the demodulating photodetector DEM. Since the demodulator typically has two outputs (0° and 180° demodulated photocurrent), the same readout circuit is duplicated in every pixel. The operation starts with resetting the integration capacitance to initial volta...

AI Technical Summary

Benefits of technology

The proposed solution effectively suppresses background light while maintaining high conversion gain for differential ToF signals with low power consumption and area, improving sensitivity and reducing noise, especially in low-light conditions.

Problems solved by technology

The magnitude of the background signal can greatly exceed the useful modulated signal magnitude, which puts challenging requirements on the pixel dynamic range.

First, the use of a big integration capacitor introduces reset kT / C noise and lowers the pixel's conversion gain significantly degrading the Time-of-Flight sensor performance in darkness.

Further, splitting a frame into a plurality of microframes increases the requirements on the ADC bandwidth.

It also increases the power consumption and indirectly degrades (through increased ADC noise and higher on-chip temperature) the dark performance of the sensor.

Low conversion gain increases the readout noise adversely affecting pixels with lower signal amplitude due to e.g. lower object reflectivity.

It provides only a partial solution to the problem since practically the selection of low or high gain is made globally per the whole array of pixels—thus, in a high sunlight conditions, pixels receiving low signal from low reflectivity objects would be operating in a low gain mode with deteriorated performance.

This approach has a significant disadvantage, namely that the practical implementation of the integration capacitor always has parasitics associated with at least one of its terminals.

Every time the common mode reset occurs in such a system the charge stored on these parasitic capacitors is lost and the ToF signal gets significantly deteriorated.

The effectiveness of the background light subtraction in such a circuit is greatly reduced by unavoidable parasitic capacitance.

However, due to a high mismatch of the current mirrors biased by low photocurrents, this solution turns out to be impractical.

However, the power consumption of the operational amplifier puts limits on achievable resolution of ToF sensor array.

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